In automotive powertrain control, a transition between a driven and an undriven transmission gear, such as between drive or reverse and park or neutral, is commonly referred to as a garage shift maneuver. An undesirable engine speed change often occurs during a garage shift maneuver due to application of or removal of the substantial torque load of the driven vehicle wheels from the engine. The speed change may simply be an annoyance to the powertrain operator or may significantly affect engine performance. The step change in torque load during a garage shift maneuver provides a significant control challenge for conventional powertrain control systems which attempt to reject the change in torque load to maintain a substantially constant engine speed throughout a garage shift maneuver.
Engine speed control during a garage shift maneuver has been proposed. The proposed control varies engine speed during a garage shift maneuver in response to sensed change in engine speed and in engine intake manifold absolute air pressure (MAP). When a significant change in engine speed is accompanied by a significant MAP change under such proposed control, engine fueling rate, intake air rate or ignition timing is adjusted as a function of the engine speed change and perhaps further as a function of the MAP change to vary engine output torque in direction to minimize such change. The difficulty with this approach is that it is reactive. A significant and officious engine speed deviation away from a desired speed may be required before any action is taken to reduce such speed change. An unpleasant engine speed deviation away from a target engine speed may be unavoidable during garage shift maneuvers under such proposed control. Powertrain stability and powertrain operator confidence may suffer under this reactive approach.
It has further been proposed to control engine speed during a garage shift maneuver by controlling the hydraulic pressure applied across transmission shift control lines to vary the time of transmission gear shift. Line pressure is proposed to be increased or decreased as a function of an estimated prior garage shift timing error. Such proposed control requires engine speed to be stable and within a prescribed speed range at the onset of the garage shift maneuver. If such conditions are not met, as is frequently the case during a garage shift maneuver, an undesirable engine speed deviation away from a target engine speed may occur during the garage shift maneuver.
A hydrodynamic converter (hereinafter a torque converter) is generally known to be applied as a coupling between an engine and a transmission, providing well-established torque multiplication and hydrodynamic damping benefits. The torque converter includes a pump which rotates with an engine output shaft and a turbine which rotates with a transmission input shaft. The pump drives transmission fluid in the torque converter assembly which drives the turbine. In torque multiplication mode, a positive slip (rotational speed difference) is present between the pump and the turbine providing for a torque multiplication across the torque converter. In high efficiency mode, virtually no slip is present between the pump and the turbine, providing for a hydrodynamic clutch function. During a garage shift maneuver, the rate of rotation of the turbine (turbine speed) changes rapidly due to a step change in transmission torque load. The load is passed across the torque converter from the turbine to the pump and is then applied as an engine output torque load change which can, under the prior art control, perturb engine speed substantially away from a target engine speed.
FIG. 1A illustrates turbine speed change under two representative garage shift maneuvers. FIG. 1B illustrates a corresponding (undesirable) engine speed change for the same two garage shift maneuvers. Curve 100 represents turbine speed for a powertrain in a neutral or park transmission gear with substantially no load applied across the torque converter such that engine speed (curve 120 of FIG. 1B) and turbine speed are substantially at a target rate of rotation. At time t1, a garage shift maneuver is initiated in which the transmission is shifted from the neutral or park gear into a driven gear (either a drive gear or a reverse gear). The application of a transmission load immediately causes a significant decrease in turbine speed toward zero as illustrated by curve 102 of FIG. 1A, while engine speed initially remains substantially constant and remains largely unaffected throughout a period of time .delta.t during which the change in load is transferred across the torque converter to the engine output shaft. Following time .delta.t, the load change is reflected to the engine output shaft and engine speed begins a sharp decrease as illustrated by curve 122. Prior art engine speed control procedures may reactively compensate the engine speed drop well after the time delay .delta.t and drive engine speed back toward the target engine speed. Engine speed control may later stabilize following a settling time ts.
At time t2, a second garage shift maneuver is initiated in which the transmission is shifted from a drive gear (drive or reverse) into park or neutral which corresponds to a step removal of a torque load. The torque load removal immediately causes a significant increase in turbine speed from zero (or any initial speed) toward a final speed along curve 104 (FIG. 1A), while engine speed initially remains constant. Following a period of time .delta.t1 after time t2 during which the change in load is transferred across the torque converter to the engine output shaft, engine speed begins a speed increase as illustrated by curve 124. Prior art engine speed control procedures may reactively compensate the engine speed increase a period of time after time .delta.t1 and drive engine speed back toward the target engine speed. Engine speed control may later stabilize following a settling time ts1. FIGS. 1A and 1B illustrate the significant delay between the start of the garage shift maneuver and the time a reactive compensator, such as the described prior art compensator, responds, to stabilize engine speed control.
It would be desirable to anticipate engine speed change during a garage shift maneuver and, under all engine operating conditions, proactively compensate the anticipated change so that engine speed may remain stable throughout a garage shift maneuver.